Power inverters (or in short inverters) are electronic devices which transform direct current (DC) to alternating current (AC). In particular, inverters play nowadays an economic and environmental role which is more and more important in the frame of transformation of DC current produced by solar panels, batteries or similar sources into AC current for domestic or industrial use as well as in electric cars.
Inverters manufactured by the Applicant for commercial and industrial companies permit saving of their critical applications by using energy stored in batteries, during distribution grid breakdown. Inverter Media™ manufactured by the Applicant already allows to reach a power density of 680 W/liter at 2 kVA.
Inverters used for example in electricity production facilities from solar energy still have a noticeable size (typically 50 liters or the size of a portable cooler). Size reduction of >10× in volume, i.e. typically shrinking down to something smaller than a small laptop would enable powering more homes with solar energy, as well as improving distribution efficiency and distances ranges reached with electrical grids. Future will thus be dedicated to more robust, more reliable and more intelligent power inverters.
In order to achieve very high power density and consequently smaller conversion systems, designers of inverter topologies had primarily to target increased efficiency and common mode (CM) noise reduction. Higher efficiency has been achieved thanks to improvements in semiconductor materials and processing, as well as in magnetic materials. Use of wideband-gap semiconductors (silicon carbide—SiC or gallium nitride—GaN) allows to improve efficiency in high frequency power converters, while the latter allow increasing switching frequency and thus reducing passive components size.
It is known that EMI noise is both in the form of conducted EMI, i.e. noise travelling along wires or conducting paths and through electronic components and in the form of radiated EMI (RFI), i.e. noise travelling through the air in the form of electro-magnetic fields or radio waves. In high-speed switching converters (frequency typically from 50 kHz to 1 MHz), most of the conducted EMI comes from the switching transistors and from the rectifiers. For preventing such EMI noise, one generally uses EMI filters made of passive components such as capacitors and inductors forming LC circuits. Conducted EMI is divided into common-mode noise (CMN) and differential-mode noise (DMN). CMN flows in the same direction in line and neutral AC power conductors, is in phase with itself relative to ground and returns to ground. Suitable CMN filter comprises inductors L100, L200 placed in series with each power line and respective Y-capacitors C100, C200 connecting each power conductors to ground (see for example CMN filter 100 in FIG. 1 in the case of a DC/AC converter). DMN exists between AC line and neutral conductors and is 180° out of phase with itself. Suitable DMN filter comprises C340 X-capacitors bridging the power lines, possibly supplemented by differential-suppression inductors L300, L400 (see for example DMN filter 101 in FIG. 1 in the case of a DC/AC converter).
Document US 2011/0026281 A1 discloses an apparatus and method for controlling the delivery of power from a DC source to an AC grid including an inverter configured to deliver power from the unipolar input source to the AC grid and an inverter controller. The inverter includes an input converter, an active filter, and an output converter. The inverter controller includes an input converter controller, an active filter controller and an output converter controller. The input converter controller is configured to control a current delivered by the input converter to a galvanically isolated unipolar bus of the inverter. The output converter is configured to control the output converter to deliver power to the AC grid. Additionally, the active filter controller is configured to control the active filter to supply substantially all the power that is delivered by the output controller to the AC grid at a grid frequency.
Document J. W. Kolar et al, “PWM Converter Power Density barriers», Fourth power conversion conference, 2-5 Apr. 2007, Nagoya (Japan), IEEE (2007), pages 9-29, teaches that in high power density power converters the use of wide band-gap semiconductor switches allows for high size and cost reduction of the passive components.
Document R. Gonzales et al, “Transformerless Single-Phase Multilevel-Based Photovoltaic Inverter», IEEE Trabsactions on Industrial Electronics, Vol. 55, No. 7 (2008), pp. 2694-2702, teaches that the elimination of the output transformer from grid-connected photovoltaic (PV) systems not only reduces the cost, size and weight of the conversion stage but also increases the system overall efficiency. They propose a new high-efficiency topology for transformerless systems, which does not generate common mode currents and topologically guarantees that no dc is injected into the grid. The proposed topology has been verified in a 5 kW prototype.
Document M. Liserre et al, “An Anti-Islanding Method for Single-Phase Inverters Based on a Grid Voltage Sensorless Control”, IEEE Transactions on Industrial electronics, Vol. 53, No. 5 (2006), pp. 1418-1426, and document Yasser Abdel-Rady Ibrahim Mohamed et al, “Adaptative Discrete-Time Grid-Voltage Sensorless Interfacing Scheme for Grid-Connected DG-Inverters Based on Neural-Network Identification and Deadbeat Current Regulation”, IEEE Transactions on Power Electronics, Vol. 23, No. 1 (2008), pp. 308-321 disclose the use of sensorless state observer current controllers in the field of power inverters.
JP 5 300775 B2 discloses common-mode reduction architecture for a converter connected via a long (>1 km) shielded cable to a jet-fan motor, wherein the Y-capacitors of the common-mode reduction LC-filter are referenced to a shielding of shielded cable, being at a reference potential insulated from earth, i.e. there is no connection between the shield and the earth symbols. Referencing a common mode noise filter to a shielding being at reference potential, said shielding being insulated from earth when implementing EMC shielding is also known from WO 2015/125107 A1.
Finally, T. Friedli et al, “Comparative Evaluation of Three-Phase AC-AC Matrix Converter and Voltage DC-Link Back-to-Back Converter Systems”, IEEE Transactions on Industrial Electronics, Vol. 59, No. 12 (2012), pp. 4487-4510, Parthasarathy Nayak et al, “Study of the Effects of Parasitic Inductances and Device Capacitances on 1200 V, 35 A SiC MOSFET Based Voltage Source Inverter Design”, 2014 IEEE International Conference on Power Electronics, Drives and Energy Systems (PEDES), IEEE 2014, pp. 1-6 and P. Kumar et al, “Soft Computing Techniques for the Control of an Active Power Filter”, IEEE Transactions on Power Delivery, Vol. 24, No. 1 (2009), pp. 452-461 are background art documents in the field of the present invention.